Note: Descriptions are shown in the official language in which they were submitted.
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A MATERIAL FOR FORMING A DENTAL COPING IN THE PREPARATION OF
A DENTAL RESTORATION
FIELD OF THE INVENTION
This invention relates to a dental material composition for use in forming
a porous metal structure and dental coping in the preparation or repair of a
dental restoration.
BACKGROUND OF THE INVENTION
In crown and bridge prosthodontics, metal copings are conventionally
used to form the understructure of a dental resto'ration such as a crown and/
or
bridge. The metal coping must provide the required structural strength and
rigidity necessary for the restoration to resist the forces of mastication
when
chewing food. In a ceramic-to-metal dental restoration the metal coping is
covered with a fired-on coating of porcelain or acrylic for purposes primarily
of
aesthetics. Desirably, the metal coping should also provide a biocompatible
relationship with the gingival tissue at the margin of the dental restoration.
Traditionally the dental coping was formed from cast metal using a
conventional casting operation. Over the past twenty years a substitute
procedure using a base composition of metal particles to form the dental
coping
has been introduced to the dental profession and has become widely accepted
as a replacement for the traditional cast coping. In this substitute procedure
the
base composition of metal particles is shaped and/or molded over a die into a
desired configuration and then heat treated into a porous structure which
forms
the dental coping upon the incorporation of a filler material pursuant to a
subsequent heat treatment operation. The addition of filler material is
required
to solidify the porous structure into a solid mass representative of the
finished
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dental coping before placement of the dental coping in the mouth of the
patient.
The dental material of choice for forming a dental coping in this manner is
"Captek", a registered trademark of Precious Chemicals Inc., which is a
composition comprising noble metal particles as taught and described in US
Patent No.'s: 4,742,861; 4,814,008; 4,990,394; 5, 234, 343 and 5,336,091 the
disclosures of which are herein incorporated by reference.
As described in the aforementioned patents a dental coping is formed
from a base material composition comprising a combination of high fusing
temperature metal particles and low fusing temperature metal particles
combined in a matrix which should preferably include a volatile binder. Other
minor constituents may be included such as particles of activated carbon. The
high and low fusing temperature metal particles are selected to provide a
melting
characteristic which lies respectively above and below the heat treatment
temperature to which the metal composite is to be subjected for forming the
dental coping. Accordingly, the low fusing temperature metal particles should
have a melting temperature to cause the particles of low fusing metal to melt
at
or below the selected heat treatment temperature whereas the high fusing
temperature metal particles should have a melting temperature above the
selected heat treatment temperature so that they essentially will not melt
during
heat treatment. The volatile binder may be composed of a material such as
dental wax which should vaporize during heat treatment. The heat treatment
process may involve more than one heat treatment step using different heat
treatment temperatures or may involve use of a graduated temperature. In this
instance the high and low fusing temperature metal particles are still
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distinguished from one another by the fact that the low fusing temperature
metal
particles should all melt during heat treatment whereas the high fusing
temperature metal particles essentially do not melt during heat treatment.
Since
heat treatment occurs over a given time interval which is variable little or
no
melting of the high fusing metal particles should occur despite the selected
time
interval of heat treatment or the porous structure formed during heat
treatment
will shrink and result in failure.
Although this relatively new dental material and procedure for forming a
dental coping has received wide acceptance in the dental profession a
considerable amount of expertise is presently required of the dental
laboratory
technician or dentist in using the material to properly shape the material on
the
die particularly at the margin and to heat treat the material. The necessity
to
acquire such expertise has inhibited many laboratories and single
practitioners
from using this material and procedure on a consistent basis. Obviously, a
dental
material which would require less expertise for forming the dental coping
would
increase the desirability for using such material in the preparation of a
dental
restoration and/or to repair existing restorations.
The dental composition of the present invention is a modification in the
formulation of the matrix of metal particles taught in the aforementioned
patents
and represents a substantial improvement in that the material composition of
the
present invention allows the material to be shaped and heat treated with much
less expertise and little or no follow up supervision.
SUMMARY OF THE INVENTION
The dental material of the present invention is a composition comprising
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a composite of metal particles in a matrix of metal particles including low
fusing
temperature metal particles and at least between 35% to 85%, preferably 65%,
medium fusing temperature metal particles by volume with the medium fusing
temperature metal particles possessing a melting characteristic during heat
treatment for causing the medium fusing temperature metal particles to
partially
melt over a heat treatment time of between 1 to 10 minutes, preferably 3 to 6
minutes, at between 1000 C and 1200 C such that all of the low fusing
temperature metal particles have melted and by the end of heat treatment at
least substantially all of the medium fusing temperature metal particles have
partially melted to a limited extent wherein about 4% to 50% by volume of the
particle mass of each such medium fusing temperature metal particle has melted
with the low fusing temperature metal particles representing the remainder of
the
composition of metal particles or including a relatively minor percentage of
high
fusing temperature metal particles. The medium fusing particles and high
fusing
temperature metal particles, if included, should be thin and of non-spherical
geometry and preferably in the form of thin flakes having one or more jagged
edges.
By limiting the extent to which all or substantially all of the medium fusing
particles melt during heat treatment so that no more than 50% by volume of
each
such particle melts and preferably between 10 to 25% by volume of each such
particle melts a porous skeleton structure will form which will not shrink as
a
result of heat treatment and will require little or no training to use.
However it is
essential to the present invention that all or substantially all of the medium
fusing
particles at least partially melt during heat treatment to the extent that
above
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about 4% by volume of each such particle has melted but less than 50% of its
volume and preferably between 10 to 25% with 10 to 15% being optimum. The
controlled partial melting of the medium fusing particles permits the material
to
be shaped or molded to the die even at the margins without shrinkage. The
melted portion of each of the medium fusing particles combine and fuse with
the
melted low fusing particles to enhance the properties particularly the
temperature stability of the porous structure formed from the non-melted
portion
of the medium fusing particles. The composition can comprise medium
temperature fusing particles alone or in combination with high fusing
particles.
Moreover, more than one composition of medium fusing particles may be used
to provide greater control over the partial melting characteristic of the
total
medium fusing particle content in the dental material composition.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is a moldable dental material which, in general, is
taught in US Patent No's. 4,742,861; 4,814,008; 4,990,394; 5, 234, 343 and
5,336,091 the disclosures of which are herein incorporated by reference for
forming, repairing or restoring dental restorations using any conventional
stone,
metal or polymer non-refractory or refractory working die.
More specifically the dental material of the present invention is a
moldable composition comprising medium fusing temperature metal particles
alone or in combination with high fusing temperature metal particles, low-
fusing
temperature metal particles and a volatile binder. Upon heat treatment, the
binder should vaporize forming a porous, three dimensional skeleton or
sponge-like structure having a capillary network of multiple voids uniformly
distributed throughout the structure, with a void volume preferably above
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twenty percent (20%), and up to a void volume of eighty percent (80%).
The binder may be any suitable vehicle which will vaporize upon heat
treatment, to facilitate the formation of a porous structure. The preferred
binder is
composed substantially or entirely of wax, with the remainder, if any, of an
organic
or hydrocarbon compound to control the malleability of the dental material.
The
term "wax," for purposes of the present invention, means any natural wax,
mineral
wax, or organic wax, or combination thereof. The concentration of the binder
is
preferably high enough to assure a void volume of at least twenty percent
(20%).
In addition to the metal particles and binder, the dental material should
preferably contain a small amount of carbonaceous particles of preferably
activated carbon in an amount of between five-thousands of one percent
(0.005%) of the weight of the metal mixture, to about two percent (2%) of the
weight of the metal mixture, with 0.05 wt.% to 0.5 wt.% being preferred.
The component of medium fusing temperature metal particles is critical to
the composition of metal particles in the dental material of the present
invention.
The medium fusing temperature metal particles are selected primarily or
entirely
from noble metals and will possess a melting characteristic during heat
treatment within a heat treatment temperature range of between 1000 C and
1200 C for causing all or substantially all of the medium fusing temperature
metal particles to partially melt over a heat treatment time of between 1 to
10
minutes, preferably 3 to 6 minutes, such that at the end of heat treatment the
medium fusing temperature metal particles have partially melted but only to a
limited extent. To satisfy this requirement it is essential to melt between
about
4% to 50% by volume of the particle mass of each medium fusing temperature
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metal particle during heat treatment. The extent of melting of the medium
fusing
particles can be determined and verified with relative accuracy using a scan
electron microscope having at least a 3 to 5 nanometer resolution and
preferably
a resolution of between 1 to 2 nanometers. A porous skeleton structure of high
density will be formed by the residual non-melted portion of the medium fusing
particles. If the medium fusing particles melt in excess of 50% by volume the
skeleton structure will shrink during heat treatment. In general, a structure
which
shrinks during heat treatment is considered detrimental and preferably no
shrinkage at all should take place during heat treatment. This is achieved in
accordance with the present invention by limiting the amount of melting of the
medium fusing particles to between 10 and 25% by volume of each medium
fusing particle. Conversely, some partial melting of each of the medium fusing
particles during heat treatment has been found essential for causing the
melted
portion of the medium fusing particles to combine with the melting low fusing
particles and form a melt mixture which stabilizes the formation of the porous
structure even at the margins. Partial melting requires melting of the medium
fusing particles to the extent of at least about 4% by volume, preferably
between
and 25% by volume and optimally between 10 to 15%. By stabilizing the
formation of the porous structure the dental material does not require any
degree
of skill to apply and mold to the die prior to heat treatment.
To the extent that some of the medium fusing particles do not melt these,
in essence, are equivalent to high fusing temperature metal particles.
Alternatively, high fusing temperature metal particles may be included in the
mixture of metal particles but should be limited to no more than about twenty
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(20)% by volume of the mixture in the material composition and preferably less
than 10% by volume.
At least 35% to 85% by volume of the mixture of metal particles in the
material composition should consist of medium fusing temperature metal
particles with the remainder consisting of low fusing temperature metal
particles
or a combination of low fusing temperature metal particles and a limited
amount
of high fusing temperature metal particles. In general between 30 to 60% of
the
mixture of particles will consist of low fusing temperature metal particles
but
preferably no more than about 50% low fusing temperature metal particles.
As indicated above the composition of the medium fusing temperature
metal particles should be selected from noble metals and preferably platinum
and palladium in any desired proportion to one another from zero to one
hundred
percent in addition, gold may be primarily added although other metal
constituents to a smaller degree may also be added such as Ag, Cu, Mg, Al, Zn
and Re. Moreover, other metals of the platinum group of elements of the third
and fourth group of elements may be added. It is the proportion of gold and
the
other constituents which will control the melting characteristic of the medium
fusing temperature metal particles such that each of the medium fusing
particles
will partially melt to within a controlled degree as explained heretofore.
Alternatively, more than one medium fusing particle composition may be used to
provide greater control over the partial melting characteristic of the total
medium
fusing particle content in the dental material composition of the present
invention.
For example, two or more medium fusing particle compositions may be used to
form the total medium fusing component of the mixture of metal particles with
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one of the compositions constituting a major or primary portion of between 75
to
95% of the total medium fusing component and the others representing a minor
or secondary portion. The major portion may contain more gold than the other
secondary portions and/or the secondary portions may contain harder materials
such as titanium, rhodium, strontium and possibly stainless steel can be
judiciously added to provide a different melting percentage for the secondary
portions relative to the primary portion over the given time period within the
heat
treatment period.
The medium fusing particles should have an irregular shape preferably in
the form of flakes such as platelets and should be thin and flat. However, it
is
preferred that the thin flakes also have jagged edges. The existence of jagged
edges provides greater interlocking or interleaving of particles during heat
treatment.
The low fusing particles are composed preferably of gold or a gold alloy,
with gold as the major constituent. The preference for gold as the major
constituent of the low-fusing component is based on its known characteristics
of
workability, biocompatibility, non-oxidizing properties, and color. The low-
fusing
metal particles must melt during heat treatment for forming the porous
structure.
It should be understood that heat treatment may occur in stages at
different temperatures over a temperature range of between 1000 C and
1200 C. However, the medium fusing particles should possess a melting
characteristic during heat treatment over a heat treatment time of between 1
to
minutes, preferably 3 to 6 minutes, such that at the end of such heat
treatment substantially all of the medium fusing temperature metal particles
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have partially melted as explained above.
Following the formation of the porous structure a filler material is melted
into the voids of the porous structure to solidify the structure and form the
finished dental coping. The filler material may be any suitable ceramic or
metal
composition, preferably a precious metal composition. The filler material may
also be formed of a matrix of particles mixed with a wax binder or mixed with
a
liquid vehicle to help spread it. The filler material binder may have a
composition
and concentration similar to the composition and concentration of the binder
used in the dental material to form the porous structure.
